Although it had won a hegemony over the Greek city-states from its leadership in the Persian Wars, the Spartan-led Peloponnesian League feared the growing power of the Athenian empire and worsened relations by repeated diplomatic affronts and demands. Wanting to deny any future Persian invasion a base from which to operate, Sparta had urged Athens, along with other Greek cities, to refrain from rebuilding their walls. However, suspecting a Spartan ploy and having already begun the work of construction, Athens employed subterfuge to delay the wheels of diplomacy until she could finish them. In 464 BC, suffering another Helot rebellion and failing to make progress in the siege against their stronghold Ithome, Sparta had asked for Athens' aid along with its other allies. But after a "considerable force" arrived from Athens under the command of Cimon, Sparta, fearing the "unorthodox" politics of Athens and the possibility of her supporting the enslaved Helots rather than fighting them, sent the Athenian contingent home while keeping on the rest of her allies. Deeply offended by these Spartan interferences and insults, Athens was increasingly willing to support discord within the Peloponnesian League and took Megara into its protection during its border dispute with the Spartan-allied Corinth, leading to open war with Corinth but not Sparta herself.
The battle
When the Phocians made war on the cities of Doris—the traditional homeland of Doric Greeks—the Doric Sparta sent a relief force under the command of Nicomedes, son of Cleombrotus, acting as regent for his under-age nephew, King Pleistoanax. An army of 1,500 Spartan hoplites with 10,000 of their allies entered Boeotia and compelled the submission of Phocis. Athens, already contemptuous of Spartan treatment and now suspecting her of negotiating with factions within the city to undermine democracy and prevent the construction of the Long Walls, maneuvered to cut off the Spartan army isolated in Boeotia. Facing either transport through waters controlled by the Athenian navy or a difficult march through the Geraneiamountain passes held by Athenian soldiers supported from Megara, the Spartans decided to wait either for the opening of a safe route home or an outright Athenian assault. Meeting the Spartans at Tanagra, Athens fielded "their whole army, supported by 1,000 troops from Argos and by contingents from their other allies, making up altogether a force of 14,000 men." Although both sides sustained "great losses," the Spartans were victorious and now able to return home through the mountain passes of the Isthmus. The Athenian politician and general Cimon, who had been exiled from Athens 3 years prior, came to the Athenian camp to offer to fight but was sent away
Aftermath
Two months later, the Athenians regrouped and defeated Thebes at the Battle of Oenophyta and took control of Boeotia, taking down the wall the Spartans had built. With the victory the Athenians also occupied Phocis, the original source of the conflict and the Opuntian Locris.
